The invention relates to a secondary-electron detector for the analysis of irradiated samples for scanning electron microscopes and microprobes, including an electron collector arranged along a curved or angular path and an electron accelerator, a scintillator, a light guide and a photomultiplier.
If a secondary-electron detector of conventional design is used for the observation of highly radioactive material, for example irradiated nuclear fuel, the surface of the scintillator will be destroyed within a few days. Furthermore, the impact of .gamma. rays and high-energy .beta. rays on the scintillator and on the dynodes of the photomultiplier (SEM) causes a relatively high level of background noise, whereby clear secondary-electron imaging cannot be achieved.
A secondary-electron detector for shielded scanning electron microscopes and shielded microprobes is known in the prior art, wherein the scintillator, the light guide and the SEM are arranged behind a thick-walled heavy-metal shield to reduce radiation damage, and a curved or angular electron guide tube comprising cylindrical electrodes is used to guide the secondary electrons to the scintillator which is shielded with respect to the sample. The voltage applied to the scintillator, which is in the order of magnitude of 10 kV, is used to separate the secondary electron signal from the background noise. This bias voltage primarily deflects the secondary electrons from their linear trajectory and causes them to impinge upon the scintillator.
This known apparatus has three principal defects. First of all a large part of the electron guidance tube is exposed to the .beta. and .gamma. rays which impinge upon the electrodes and produce fluorescence electrons, at least some of which intensify the background noise. Secondly, the electron guide tube not only guides secondary electrons to the scintillator but also fluorescence electrons with an energy which is less than the bias voltage of 10 kV, which degrades the signal-to-noise ratio. Finally, the detector is to a large extent endangered by contamination from volatile fission products of irradiated nuclear fuels, for example caesium, and such contamination causes a considerable intensification of the background noise after a relatively short operating time.